docs: def bounding_ellipses(self):
          """
          Compute the semimajor axis of the two ellipses that bound the
          annulus where integrations take place.
  
          Returns
          -------
          sma1, sma2 : float
              The smaller and larger values of semimajor axis length that
              define the annulus bounding ellipses.
          """
  
          if (self.linear_growth):
              a1 = self.sma - self.astep / 2.
              a2 = self.sma + self.astep / 2.
          else:
              a1 = self.sma * (1. - self.astep / 2.)
              a2 = self.sma * (1. + self.astep / 2.)
  
          return a1, a2